It is recalled that in the field of rotorcraft, in particular helicopters or the like, it is common practice to drive the rotor by means of a plurality of engine members, in particular by means of a pair of engines. It is also recalled that at least one of the engine members is also used to drive accessories of the rotorcraft, such as an alternator or a compressor, for example. At this stage of the description, it should be observed that the number and the kind of such accessories are not limiting on the scope of the present invention.
A general problem posed lies in the dual purpose of the engine members of the rotorcraft, i.e. both driving the rotor, and for at least one of them, also driving the accessories. This problem is made particularly difficult to solve in that it is common practice to drive the accessories independently of the rotor while the rotorcraft is on the ground, either while preparing to take off or while waiting, in particular. To this end, it is common practice for one of the engine members to be allocated to driving the accessories, while the connection between the rotor and the set of engine members is interrupted.
More precisely, the engine member for driving the accessories is in communication firstly with a main drive shaft for the rotor via a selective drive member, such as a positive clutch mechanism or the like, and secondly it is in communication with a secondary shaft for driving the accessories. These dispositions are such that when the selective drive member is moved, the engine member for driving the accessories can either drive the accessories alone, or else it can drive both the accessories and also the rotor.
In particular during a stage while the rotorcraft is being prepared, prior to setting the rotor into rotation, the connection between the engine member for the accessories and the main shaft is interrupted, with only the accessories being driven by this engine member. Thereafter, once this preparation stage has come to an end, the connection between the engine member for driving the accessories and the rotor is established via the selective drive member. Likewise, during a waiting period for the rotorcraft during which drive to the rotor is temporarily interrupted, the selective drive member must be moved to break the connection between the main shaft and the engine allocated to driving the accessories, while nevertheless continuing to drive the accessories.
Patents FR 2 140 107 and U.S. Pat. No. 3,782,223 disclose a device comprising a large number of parts that turns out to be complex, bulky, and relatively fragile.
At this stage in the description, it should be observed that it is also common practice to interpose rotary differential mechanisms between the engine members and the associated main shafts for driving the rotor, where said rotary differential mechanisms may be constituted in particular by freewheels or the like. These freewheels are even more important when the rotor presents significant inertia, and they serve to ensure that no harm is done due to a relative speed difference between the rotor and one or other of the main shafts for driving it in association with the corresponding engine member. Nevertheless, it is common practice to operate the selective drive member (clutch) that is interposed between the main drive shaft of the rotor and the engine member for driving the accessories, by moving it in an axial direction, and that making such an axial movement is made difficult or even impossible beyond an acceptable threshold of rotary speed difference between the rotor and the engine member allocated to driving the accessories.
To overcome this difficulty, it is common on passing from the position for driving the accessories alone to the position for driving both the accessories and the rotor, to wait for said speed difference threshold to be reached prior to moving the selective drive member and causing the rotor to be driven from both engine members. Conversely, while the rotor is being driven by both engine members, the selective drive member is moved axially, and then only the engine member for driving the accessories is maintained in operation. Since moving the selective drive member is made difficult because of the difference in speed of rotation between the engine members, in particular on going from the position for driving the accessories alone to the position for driving the accessories together with the rotor, it is commonly accepted that drive to the accessories needs to be interrupted momentarily until a sufficiently close match has been achieved between the drive speeds of the various rotary members relative to one another.
In use, it has been found that such a general organization for the transmission mechanism is not fully satisfactory, in particular because of the risk of losing accessory function during the period of waiting for the speeds to match, as mentioned above, and/or in the event of any of the engines breaking down. Interrupting drive to the accessories, even if only momentarily, is harmful since the accessories might include in particular an air conditioner or an alternator for powering memory members, so interrupting the alternator can have the consequence of losing information. Nevertheless, in particular compared with the drawback of undesirable extra weight, the practice in this field tends to ignore this lack of satisfaction.
This practice is particularly accepted given that operating constraints for accessories, and in particular for the alternator, require them to be driven in compliance with minimum and maximum speed thresholds, thereby tending to make the transmission mechanism structure significantly more complex and making it inconceivable to achieve a structure that would enable those thresholds to be guaranteed while driving accessories under all circumstances.